The present invention relates to an image pickup apparatus which enlarges the dynamic range of image data to be processed. Furthermore the present invention relates to a method of processing image data with a large dynamic range.
The dynamic range of an image signal generated by an image sensor in CCD- or CMOS-technology is limited by its noise level on the one hand, and the saturation voltage on the other hand. For a CCD-sensor typically a dynamic range of 74 dB can be obtained which is sufficient for most applications. For applications with a very large contrast ratio, however, e.g. where outdoor scenes with bright sunlight are involved, a greater dynamic range is required in order to obtain images of a satisfactory quality. For example the contrast ratio in a sunny outdoor scene can be as high as 1000× (60 dB). For the lowest level in that scene the signal to noise ratio (SNR) needs to be 40 dB in order to achieve an acceptable quality. Therefore the total dynamic range should be about 100 dB. There were already some solutions proposed in literature to achieve this goal. A very popular idea known from the state of the art (U.S. Pat. No. 4,647,975 A1) is a double exposure solution where two pictures are taken with a short time interval in between. One picture is taken with a short exposure time and the second picture is taken with a longer exposure time. Combining the two pictures results in a good SNR in the dark parts of the picture due to the long exposure time of the second picture. In addition there is almost no saturation in the bright parts of the picture due to the first picture taken with a short exposure time. The ratio of the exposure times varies from 4 to 32.